Part 37

A little more than four years after the opening of the Forth Bridge, in June 1894, another great enterprise which had been commenced eight years before, was inaugurated by the Prince and Princess of Wales as representatives of Her Majesty the Queen. This was the Tower Bridge, which not only is one of the most important public works of the century, but one that presents features of interest and novelty that have never before been combined in any single structure. The want of an adequate communication between the shores of the Thames eastward of London Bridge had long been felt, and was for years a subject of serious consideration for the Metropolitan authorities. The congested state of the traffic across London Bridge has often furnished a spectacle for the sight-seer, and figures are not wanting to show that the number of foot-passengers alone who daily traverse that bridge, which altogether is only 54 feet wide, would be equal to the whole population of many considerable cities: for in 1882 a count showed the daily average of pedestrians to be 110,525, while the number of vehicles was 22,242. There was much difference of opinion as to the best method of providing the required means of communication; but there was an almost universal agreement as to its position being selected just eastward of the Tower of London. The map of the districts connected by the Tower Bridge which is given in Fig. 147_d_, will show a reader who has any acquaintance with London the suitability of the site. The problem of traversing the river at this point involved complex conditions as affecting the vehicular traffic and the navigation, and many different schemes were proposed and examined, comprised under the three heads of bridges, tunnels and ferries. But a ferry is always an imperfect means of communication, liable to accidents and interruptions from fogs, and in severe weather from ice, rendering the transit impossible for sometimes many days together. A tunnel beneath the river would, of course, leave the navigation without impediment, but among its special disadvantages are the great expense of construction and maintenance, for it has been found that tunnels beneath waterways are very costly in both respects. Besides, there would have to be long inclined approaches at each end, and the cost would be enormously increased by the amount of valuable land these would occupy. It was indeed proposed that the tunnel should be provided instead with hydraulic lifts at each end, like those often found in connection with the sub-ways at railway stations; but such would have to be of Brobdignagian dimensions, and would daily entail heavy expense. Then, as regards the bridges, schemes of various kinds were proposed, some even bridging the whole 850 feet width of the river at a single span, but all distinguishable by these important characteristics: they either provided a high level roadway which requires long inclines to reach it, but permitted lofty-masted ships to pass under it; or, on the other hand, the roadway was to be made at a low level with a clear headway above the water of moderate height. While avoiding the inclined approaches, this plan would either prevent fully rigged vessels passing to the wharves above the bridge, or some part of the structure would have to open or swing aside, that the ships might pass through the opening, thus completely interrupting the pedestrian and vehicular traffic for the time, with an amount of inconvenience that may be imagined when, as often happens, twenty large ships or more might pass in the course of a day, each causing a stoppage of five minutes in the road traffic. Nor would it be without risks that large vessels could pass through a comparatively narrow opening in a strong tide-way. Plans for sub-ways, for high level roadways and for low level roadways, were examined by Parliamentary Committees when powers to construct the works were successively applied for by the Metropolitan authorities, and much valuable evidence having been given, such objectionable features of each scheme as have been already referred to were duly noted. At length in 1878, Mr. Horace Jones, the late architect to the City of London, in a report on the various projects, suggested the general plan on which the present bridge is built, and this having been approved of by the Common Council, steps were taken to obtain Parliamentary powers to raise the necessary capital and to proceed with the works; but, for various reasons, it was not until 1885 that the Act authorising the undertaking was passed. In the meantime Mr. John Wolfe Barry was appointed engineer of the structure, while Mr. Jones was to superintend the architectural details; but after having received the honour of knighthood in 1885, he died in the same year; and Mr. Barry, reconsidering the joint design, introduced some new features and somewhat modified the architectural expression of the structure. One striking point of originality about the Tower Bridge is that while it is essentially an iron and steel construction as much as the Forth Bridge, the heavy stiff metal-work is encased in masonry of elegant and appropriate architectural design, by which the general desire that the bridge should harmonize so far as might be, with the ancient historical fortress it adjoins, has been happily realised. Then again, by the ingenious engineering, the public have the advantage of a low level roadway, while the largest vessels may pass freely through a wide space without risk. These apparently incompatible advantages have been obtained by the adoption of what is the _bascule_ principle on a hitherto unattempted scale. _Bascule_ is a French engineering term, which is probably less familiar to most of our readers than the thing itself. It is applied to the platform of a draw-bridge which turns as the lid of a box does on its hinges, to afford a passage over the stream or moat when it is horizontal, and when drawn up vertically denies such passage. Smaller _bascule_ bridges on exactly the same plan as in the Tower Bridge may often be seen in places having docks or canals, such as Hull, &c. In these a flap or platform is let down from each side from the vertical position, in which the water-way is open until the free edges meet together to form the roadway. These platforms turn on horizontal pivots, and are counterpoised by loads of stone or metal, so that they are without difficulty raised and lowered by a winch or handle that turns a cogged pinion engaging the teeth of a large quadrant.

The following general description of the Tower Bridge is mainly abstracted from a very full and excellent account of it drawn up in 1894 by Mr. J. E. Tuit, engineer to Sir W. Arrol & Co., the contractors, in which are embraced the whole of the technical details of the structure. The map, Fig. 147_d_, shows the site of the bridge and its approaches, of which the northern one begins close to the mint and passes along the east side of the Tower of London to the northern abutment. This approach is formed of a series of brick arches, and is nearly 1,000 feet long and 35 feet wide in the roadway, with a footpath 12½ feet wide on either side of it. The incline is only a rise of 1 in 60, but the southern approach is slightly steeper, namely, 1 in 40 leaving the street level at Tooley Street. At each abutment there are also stairs connecting the banks of the river with the roadway of the bridge. The width of the river between the two abutments is 880 feet, and this is divided, as shown in Fig. 147_e_, into two side spans, each 270 feet wide, and one central span of 200 feet clear, making together 740 feet, the river piers, each of which is 70 feet wide, completing the total span. The clear headway above high water, when the _bascules_ or leaves are down, is, in the middle span, 29½ feet in the centre, but only 15 feet at the ends; but when the leaves are raised for ships to pass, it is about 143 feet. The headway at the shore sides of the piers is 27 feet, but this is lessened to 23 feet and 20 feet at the north and south abutments respectively. The roadway and footpaths are continued along the side spans of the same width as on the approaches, but over the central span the road is 32 feet, and each footway 8½ feet wide. The river piers are said to be the largest in the world of the same kind, and their great area was necessitated by the nature of the London clay on which they rest, which was found incapable of bearing a load much exceeding four tons per square foot without some risk of undue settlement.

The part of the piers below the bed of the river is formed of concrete, while the upper part is brickwork, set in cement and faced with Cornish granite. Upon each of the river piers rest four octagonal columns, built up of flat steel plates, connected together at their edges by splayed angle-bars. The columns are 120 feet high, and 5½ feet in diameter; those on each pier are securely braced together, at certain stages also by plate girders, 6 feet deep, to form a floor or landing, and the tops of the columns are similarly joined together. At the height of 143 feet above high water there are two footways, each 12 feet wide and 230 feet long, carried on girders over the central span, and supported by the columns on each pier. It must be noted that all the roadway, and, in fact, all the practical and useful structure of the bridge, depend upon the steel-work alone, which is supported mainly by the eight octagonal columns just mentioned. The architectural features, which so appropriately clothe all the steel columns, are added for æsthetic considerations, and their masonry takes no part in bearing the weights and strains of the structure. Indeed, the stone-work of the towers is carefully separated from the columns, which were covered with canvas while the masonry was built round them, and spaces were left at every point where compression of the steel-work would bring weight upon the stone-work. This investment of the metal-work by beautiful architecture is, as already mentioned, one of the most original features of the Tower Bridge. The view of the work in progress, as given in Plate VIII., which is one of the many beautiful illustrations in Mr. Tuit’s book, will give the reader an opportunity of judging how much the structure gains in sightliness by the addition of the architectural features. Two hydraulic lifts are placed in each tower to convey pedestrians to and from the higher level footways, when the moving parts of the bridge are open, and stairs also are provided for the same purpose for those who prefer them to using the lifts.

Length of Bridge with its approaches 2680 feet.

Length of Northern approach 1000 feet.

Length of Southern approach 800 feet.

Width between N. and S. abutments 830 feet.

Width of central span 200 feet.

Width of side spans, each 270 feet.

Depth of River at high water under central span 33½ feet.

Depth of River at lowest tides under central span 12 feet.

Clear headway at high water when the leaves are down (varies 20 to 29½ from one part of the bridge to another) feet.

Clear headway in centre span at high water with the leaves 143 feet. raised

The side spans are really suspension bridges, but the chains have only two links, connected at the lowest point by a pin 2½ feet in diameter, while their higher ends are supported on the columns of the piers, and on similar but shorter columns on the abutments. The horizontal pulls of the chains on the piers are made to balance each other by connecting the chains to tie bars stretching across the central span, and the landward ends of the chains, after passing over the lower columns of the abutments, are securely anchored in enormous masses of concrete.

Each of the opening parts, or _bascules_, or leaves, as they may be called, consists of four girders 18½ feet apart, rigidly braced together, and connected at the pier end with a great shaft, 48 feet long and 1 foot 9 inches in diameter, which turns in massive bearings, resting upon four fixed girders. The leaf is counterbalanced on the shore side of the pivot shaft by 350 tons of lead and iron; the short leverage of the centre-weight and small space available for it required the greater part of this weight to be of lead, rather than of the less expensive metal. The pivot shaft passes through the centre of gravity of the whole, so that, although the total weight is nearly 1,200 tons, no very great power is required to set it in motion, as the pivot shaft rests on rollers to diminish the friction. The power for moving the leaf is applied to toothed quadrants of 42 feet radius, of which two are fixed to the outside girders of each leaf, and are geared into cogs moved by eight large hydraulic engines, with six accumulators, into which water is pumped by two engines, each of 360 horse-power.

The total length of the bridge, including the approaches, is just half a mile, and the height of the towers from the foundations is 293 feet, so that if one of them were placed beside St. Paul’s Cathedral, it would compare with it in height as shown in the sketch, Fig. 147_f_.

_THE GREAT BROOKLYN BRIDGE._

The Clifton Bridge at Niagara Falls, which for a time had the distinction of being the longest in span of any suspension bridge in the world, has been fully described in previous pages; but more recently this bridge has been surpassed in span, and in all other respects, by a structure that immediately connects two of the most populous localities in the United States of America. The Island of Manhattan, which is occupied by the city of New York proper, has a population of nearly two millions, and a strait on its eastern side, connecting Long Island Sound with New York Harbour, alone divides it from the other great seats of population, called respectively Long Island City and Brooklyn. This channel is about ten miles long, and of a varying width, which may average three-quarters of a mile. There are many ferries between the opposite shores, and the waters are busy with steamers, sailing-boats, tugs, and craft of all kinds, engaged either in traffic with ports near at hand, or in trade with distant lands. At the southern end of this strait, near the point of its junction with New York Bay, is the narrowest part of its course, and it is here that it is crossed by the magnificent suspension bridge, known indifferently as the East River Bridge, or Brooklyn Bridge, which provides land communication between New York, with its population of two millions, and Brooklyn, the fourth city of the States in point of size, with inhabitants numbering about one million. Brooklyn is largely a residential place for persons whose daily business is in New York. It has wide, well-planned streets, many shaded by the luxuriant foliage of double rows of trees, and possesses parks, public buildings, institutes, churches, etc., on a scale commensurate with its importance.

The central span of Brooklyn Bridge, from tower to tower, is 1,595 feet, and each shore part, extending from the tower to the anchorage of the cables, is 930 feet span, while the two approaches beyond the anchorage together add 2,534 feet to the total length, which is 5,989 feet, or considerably over a mile. The centre span, it will be observed, is much greater than that of the Niagara Falls Clifton Bridge, which was less than one quarter of a mile, whereas the Brooklyn Bridge span extends to something approaching one-third of a mile, or, more exactly, a few yards longer than three-tenths. The width of the Brooklyn is another one of its remarkable features, for this is no less than 85 feet, and includes two roadways for ordinary vehicles, and two tramway tracks, on which the carriages are moved by an endless cable, worked by a stationary engine on the Brooklyn side. There is also a footpath, 13 feet wide, for pedestrians. In this structure, as in many other suspension bridges, advantage has been taken of the great tenacity of steel wire as compared with iron bars. But here the wires are not twisted in strands like ropes, but are laid straight together, and bound into a cylindrical form, each wire being 3,572 feet long, and extending from end to end of the cables, which are four in number, each calculated to bear a strain of 12,200 tons. The number of wires in each cable is very great, for instead of about the thousand of which the stranded wire cables usually consist, there are 5,296 steel wires wrapped closely round, and forming a cylinder 15¾ inches in diameter. Each wire is galvanised, that is, coated with zinc, and then coated with oil. The towers over which the cables pass are of masonry, and rise to 272 feet above high-water; their dimensions at the water level are 140 feet by 50 feet, which offsets diminish until at the top they are 120 feet by 40 feet. At the anchor structures, the cables enter the masonry at nearly 80 feet above high-water, and pass 28 feet into the stonework for connection with the anchor chains. The anchorages are masses of masonry, measuring at the base 129 feet by 119 feet, and at the top 117 feet by 104 feet, with a height of 89 feet in front and 85 feet in the rear. The weight of each anchor-plate is 23 tons. The roadway of the bridge is suspended from the cables above the buildings and streets between the towers and the anchorages. The approaches, on the Brooklyn side 971 feet, on the New York side 1,563 feet, are carried on stonework arches, which are utilised as warehouses, but where these approaches cross streets, iron bridges are thrown over. The clear headway between the centre of the roadway over the river at high-water is 135 feet, so that there is no obstruction to navigation, and the headway at the towers is 119 feet, so that the roadway rises towards the centre about 3 feet 3 inches in 100 feet. The two towers comprise more than 85,000 cubic yards of masonry, and for various purposes 13,670 tons of concrete were used. The work was commenced in January, 1870, and the first wire was carried across on 29th May, 1877. The bridge was opened to the public on the 24th of May, 1883, and the tramway four months later. The bridge was made free for pedestrians in 1891, and in 1894 the tram-car fares were reduced to five cents (2½_d._) for two journeys. In that year, 41,927,122 passengers were carried on the cars. The average number of persons daily crossing the bridge is estimated at about 115,000, although on one day (11th Feb., 1895) as many as 225,645 passengers have been carried on the cars. The cost of the work connected with this great bridge was $15,000,000 (£3,125,000).

In relation to the subject of wide-spanning bridges, the erection has been contemplated of structures which would surpass in magnitude and boldness any of those yet named. Thus, in 1894, the New York Chamber of Commerce proposed to throw across the River Hudson, which washes the western side of New York, a bridge with a clear span of 3,200 feet (six-tenths of a mile), and 500 feet clear height; and the project was declared by an eminent and experienced engineer to be quite feasible.

PRINTING MACHINES.

A volume might be filled with descriptions of the machines which in every department of industry have taken the place of slow and laborious manual labour. But if even we selected only such machines as from the beautiful mechanical principles involved in their action, or from their effects in cheapening for everybody the necessaries and comforts of life, might be considered of universal interest, the limits of the space we can afford for this class of inventions would be far exceeded. The machines for spinning, for weaving fabrics, for preparing articles of food, are in themselves worthy of attention; then there is a little machine which in almost every household has superseded one of the most primitive kinds of hand-work, and that is the sewing machine. But all these we must pass over, and confine our descriptions of special machines to a class in which the interest is of a still more general and higher character, since their effect in promoting the intellectual progress of mankind is universally acknowledged. We need hardly say that we allude to Printing Presses, and if we add a few lines on printing machines other than those which have given us cheap literature, it is because these other machines also have contributed to the general culture by giving us cheap decorative art, and in their general principles they are so much akin to the former that but little additional description is necessary.

_LETTERPRESS PRINTING._

The manner in which the youthful assistants of printers came to receive their technical appellation of “devils” has been the subject of many ingenious explanations. One of these is to the effect that the earlier productions of the press, having imitated the manuscript characters, the uninitiated supposed the impressions were produced by hand-copying, and in consequence of their rapid production and exact conformity with each other, it was thought that some diabolical agency must have been invoked. Another story relates that one of Caxton’s first assistants was a negro boy, who of course soon became identified in the popular mind with an imp from the nether world. A very innocent explanation is put forward in another tale, relating that one of the first English printers had in his employment a boy of the name of De Ville, or Deville, which name was soon corrupted into the now familiar title, and became the inheritance of this youth’s successors in the craft. Perhaps a more probable and natural explanation might be found in the personal appearance which the apprentices must have presented, with hands, and no doubt faces also, smeared over with the black ink which it was their duty to manipulate. For the ink was formerly always laid upon large round pads or balls of leather, stuffed with wool. When these balls, Fig. 149, which were, perhaps, about 12 in. in diameter, had received a charge of ink, the apprentice dabbed the one against the other, working them with a twisting motion, and after having obtained a uniform distribution of the ink on their surfaces with many dexterous flourishes, he applied them to the face of the types with both hands, until all the letters were completely and evenly charged. The operation was very troublesome, and much practice was required before the necessary skill was obtained, while it was always a most difficult matter to keep the balls in good working condition.